ALTDEV WHILE FLYING INTO AREA OF TURB.

Narrative

AT TKOF THERE WERE RAIN SHOWERS TO THE S THAT CAUSED DELAYS ON THE DEP RTES. AFTER TKOF ON RWY 9L WE WERE GIVEN A VECTOR TO 155 DEGS BY ATLANTA DEP CTL. AT 4000 FT MSL DEP ASKED US TO EXPEDITE THROUGH 6000 FT MSL FOR TFC. THE CTLR WAS VERY BUSY BECAUSE OF THE RAINSHOWERS IN THE AREA AND FREQ CONGESTION. WE HEARD THE ACFT THAT TOOK OFF PRIOR TO US (ACR) SAY THE RIDE WASN'T BAD. WE ASKED DEP CTLR ABOUT THE RIDE BECAUSE OF THE RAINSHOWERS AHEAD AND HE SAID OTHER ACFT SENT ON THE 155 DEG HDG HAD NO PROBS. WE ENCOUNTERED HEAVY RAIN AT 10000 FT MSL AND EXPERIENCED LIGHT TO MODERATE CHOP AND MULTIPLE STATIC DISCHARGES. WE ASKED FOR HIGHER AND DEPT SAID AS SOON AS WE COULD TAKE A 220 DEG HDG WE COULD START CLBING. I HAD THE RADAR TILT SET AT PLUS 5 DEGS IN ANTICIPATION OF A CONTINUED CLB AND THERE WERE NO RETURNS TOWARDS THE SW. I STARTED A TURN AND ONCE ESTABLISHED ON A 220 DEG HDG WE TRIED TO GET A CLB CLRNC BUT DEP WAS VERY BUSY; AND WE REMAINED AT 10000 FT MSL. AS WE COMPLETED THE TURN TO 220 DEGS; HEAVY RAIN APPEARED ON THE RADAR SCREEN. WE ENCOUNTERED MODERATE TURB AND A DOWNDRAFT THAT FORCED THE ACFT TO DSND. THE AUTOPLT COULD NOT HOLD THE ASSIGNED ALT AND I DISCONNECTED IT AND MANUALLY FLEW THE ACFT BACK TO 10000 FT MSL. THIS WAS A VERY INOPPORTUNE TIME TO BE HAND FLYING AND ACFT. THE ALTDEV WAS APPROX 300 FT LOW. THE INFO FROM DEP ABOUT THE RIDE AND WX WAS INACCURATE PROBABLY BECAUSE WE WERE HELD DOWN TO 10000 FT MSL FOR SO LONG. SUPPLEMENTAL INFO FROM ACN 296132: I THINK UNTIL NOW WE WERE LOOKING OVER THE RAIN (TILT TOO HIGH; SET FOR CLB). NO CLB CLRNC AND TURB BEGAN AGAIN WITH HEAVY RAIN. A SUDDEN DOWNDRAFT STARTED THE ACFT TO SINK. THE AUTOPLT DID NOT CORRECT THE ALT AND THE CAPT DISCONNECTED THE AUTOPLT AND CLBED BACK TO 10000 FT.